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Mesenchyme () is a type of loosely organised animal
embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positive reviews. Production News ...
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
of undifferentiated cells that gives rise to
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...
and
lymph vessels
lymph vessels
,
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
, and
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
.


Vertebrates


Structure

Mesenchyme is characterized morphologically by a prominent
ground substance Ground substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatom ...
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
containing a loose aggregate of
reticular fiber Reticular fibers, reticular fibres or reticulin is a type of fiber in connective tissue composed of type III collagen secreted by reticular cells. Reticular fibers crosslink to form a fine meshwork (reticulin). This network acts as a supporti ...
s and unspecialized
mesenchymal stem cells Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocyte Chondrocyt ...
. Mesenchymal cells can
migrate Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum len ...
easily, in contrast to
epithelial cell Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume o ...
s, which lack mobility, are organized into closely adherent sheets, and are
polarized
polarized
in an apical-basal orientation.


Development

The mesenchyme originates from the
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
. From the mesoderm, the mesenchyme appears as an embryologically primitive "soup". This "soup" exists as a combination of the mesenchymal cells plus serous fluid plus the many different tissue proteins. Serous fluid is typically stocked with the many serous elements, such as sodium and chloride. The mesenchyme develops into the tissues of the
lymphatic Lymph (from Latin, ''lympha'' meaning "water") is the fluid that flows through the lymphatic system The lymphatic system, or lymphoid system, is an in vertebrates that is part of the and the . It is made up of a large network of lymph, s, l ...

lymphatic
and circulatory systems, as well as the musculoskeletal system. This latter system is characterized as
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
s throughout the body, such as
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...
, muscle and
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilage
. A malignant
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumor A benign tumor is a mass of cells Cell most often refers to: * Cell (biolo ...

cancer
of mesenchymal cells is a type of
sarcoma A sarcoma is a malignant tumor Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible C ...
.


Epithelial to mesenchymal transition

The first emergence of mesenchyme occurs during
gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells) is reorganized into a multilayered structure known as the gastrula. Before gastrulat ...

gastrulation
from the
epithelial–mesenchymal transition The epithelial–mesenchymal transition (EMT) is a process by which Epithelium, epithelial cells lose their cell polarity and cell–cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells; these are multipot ...
(EMT) process. This transition occurs through the loss of epithelial cadherin,
tight junction Tight junctions, also known as occluding junctions or zonulae occludentes (singular, zonula occludens) are multiprotein junctional complexes whose general function is to prevent leakage of transported solutes and water and seals the paracellular ...
s, and
adherens junctionImage:Adherens Junctions structural proteins.svg, 350px, Principal interactions of structural proteins at cadherin-based adherens junction. Actin filaments are associated with adherens junctions in addition to several other actin-binding proteins suc ...
s on the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
s of
epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
. The surface molecules undergo
endocytosis Endocytosis is a in which are brought into the cell. The material to be internalized is surrounded by an area of , which then buds off inside the cell to form a containing the ingested material. Endocytosis includes (cell drinking) and (cell ...

endocytosis
and the
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

microtubule
cytoskeleton loses shape, enabling mesenchyme to migrate along the
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
(ECM). Epithelial–mesenchymal transition occurs in embryonic cells that require migration through or over tissue, and can be followed with a
mesenchymal–epithelial transition A mesenchymal–epithelial transition (MET) is a reversible biological process that involves the transition from motile, multipolar or spindle-shaped mesenchymal cells to planar arrays of polarized cells called epithelia. MET is the reverse process ...
to produce secondary epithelial tissues. Embryological mesenchymal cells express S100-A4 (''
S100A4 calcium-binding protein A4 (S100A4) is a that in humans is encoded by the ''S100A4'' . Function The protein encoded by this gene is a member of the S100 family of proteins containing 2 calcium-binding motifs. S100 proteins are localized in t ...
'') also known as ''fibroblast-specific protein'', which is indicative of their shared properties with the migratory adult
fibroblasts A fibroblast is a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referre ...
, and
c-Fos In the fields of molecular biology and genetics, c-Fos is a proto-oncogene that is the human homolog of the retroviral oncogene v-fos. It was first discovered in rat fibroblasts as the transforming gene of the FBJ MSV (Finkel–Biskis–Jinkins mu ...
, an
oncogene An oncogene is a gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ''birth'' ...

oncogene
associated with the down-regulation of epithelial cadherin. Both formation of the
primitive streak The primitive streak is a structure that forms in the blastula during the early stages of bird, avian, reptile, reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the developing embryo, toward the caudal or posteri ...
and mesenchymal tissue is dependent on the Wnt/β-catenin pathway. Specific markers of mesenchymal tissue include the additional expression of ECM factors such as
fibronectin Fibronectin is a high-molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena a ...

fibronectin
and
vitronectin Vitronectin (VTN or VN) is a glycoprotein Glycoproteins are protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array ...
.


Implantation

The first cells of the embryo to undergo EMT and form mesenchyme are the extra-embryonic cells of the
trophectoderm Trophoblasts (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mil ...
. These migrate from the body of the
blastocyst The blastocyst is a structure formed in the early development of mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in th ...

blastocyst
into the endometrial layer of the
uterus The uterus (from Latin "uterus", plural ''uteri'') or womb () is the main female hormone-responsive, sex organ, secondary sex organ of the reproductive system in humans and most other mammals. Things occurring in the uterus are described with t ...

uterus
in order to contribute to the formation of the anchored
placenta The placenta is a temporary fetal organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

placenta
.


Primary mesenchyme

Primary mesenchyme is the first embryonic mesenchymal tissue to emerge, and it is produced from EMT in
epiblast In amniote animal embryology, the epiblast (also known as the primitive ectoderm) is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper ...
cells. In the
epiblast In amniote animal embryology, the epiblast (also known as the primitive ectoderm) is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper ...
, it is induced by the
primitive streak The primitive streak is a structure that forms in the blastula during the early stages of bird, avian, reptile, reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the developing embryo, toward the caudal or posteri ...
through Wnt signaling, and produces
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazo ...
and
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
from a transitory tissue called mesendoderm during the process of
gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells) is reorganized into a multilayered structure known as the gastrula. Before gastrulat ...

gastrulation
. The formation of primary mesenchyme depends on the expression of
WNT3 Proto-oncogene protein Wnt-3 is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Ke ...
. Other deficiencies in signaling pathways, such as in
Nodal Nodal is a secretory protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
(a TGF-beta protein), will lead to defective
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
formation. The tissue layers formed from the primitive streak invaginate together into the embryo and the induced
mesenchymal stem cells Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocyte Chondrocyt ...
will ingress and form the
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
. Mesodermal tissue will continue to differentiate and/or migrate throughout the embryo to ultimately form most
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
layers of the body.


Neural mesenchyme

Embryological mesenchyme is particularly transitory and soon differentiates after migration. Neural mesenchyme forms soon after primary mesenchyme formation. The interaction with
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
and somite-forming morphogenic factors cause some primary mesenchyme to form neural mesenchyme, or
paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ ...
, and contribute to
somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm In all bilaterian animals, the mesoderm is one o ...

somite
formation. Neural mesenchyme soon undergoes a
mesenchymal–epithelial transition A mesenchymal–epithelial transition (MET) is a reversible biological process that involves the transition from motile, multipolar or spindle-shaped mesenchymal cells to planar arrays of polarized cells called epithelia. MET is the reverse process ...
under the influence of
WNT6 Wingless-type MMTV integration site family, member 6, also known as WNT6, is a human gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ch ...
produced by
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
to form
somites The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmentation (biology), segmented animals. In vertebrat ...

somites
. These structures will undergo a secondary EMT as the
somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm In all bilaterian animals, the mesoderm is one o ...

somite
tissue migrates later in development to form structural
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
such as
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilage
and
skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

skeletal muscle
.
Neural crest cells Neural crest cells are a temporary group of cells unique to vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdo ...
(NCCs) form from
neuroectoderm Neuroectoderm (or neural ectoderm or neural tube epithelium) consists of cells derived from ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The t ...
, instead of the primary mesenchyme, from morphogenic signals of the
neural crest Neural crest cells are a temporary group of cells unique to vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingd ...

neural crest
. The EMT occurs as a result of Wnt signaling, the influence of
Sox genes ''SOX'' genes (''SRY Testis-determining factor (TDF), also known as sex-determining region Y (SRY) protein, is a DNA-binding protein protein complex with DNA s (blue). These proteins' basic amino acids bind to the acidic phosphate groups o ...
and the loss of
E-cadherin Cadherin-1 (not to be confused with the APC/C activator protein CDH1 Cdh1 is one of the substrate Signal transducing adaptor protein, adaptor protein of the anaphase-promoting complex (APC) in the budding yeast ''Saccharomyces cerevisiae''. Functi ...
from the cell surface. NCCs additionally require the repression of
N-cadherin N-cadherin, also known as Cadherin-2 (CDH2) or neural cadherin (NCAD) is a protein that in humans is encoded by the ''CDH2'' gene. CDH2 has also been designated as CD325 (cluster of differentiation 325). N-cadherin is a transmembrane domain, transm ...
, and
neural cell adhesion molecule Neural cell adhesion molecule (NCAM), also called CD56, is a homophilic binding glycoprotein expressed on the surface of neurons A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that ...
. NCCs ingress into the embryo from the epithelial neuroectodermal layer and migrate throughout the body in order form multiple
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
(PNS) cells and
melanocytes Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the ...
. Migration of NCCs is primarily induced by BMP signaling and its inhibitor, Noggin.


Invertebrates

In some
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s, e.g.,
Porifera Sponges, the members of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. T ...

Porifera
,
Cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

Cnidaria
,
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category ...

Ctenophora
and some triploblasts (the acoelomates), mesenchyme refers to a more-or-less solid but loosely organized tissue consisting of a gel matrix (the
mesoglea Mesoglea refers to the tissue found in cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical num ...
) with various cellular and fibrous inclusions, located between the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...

epidermis
and the
gastrodermisThe gastrodermis is the inner layer of cells that serves as a lining membrane of the gastrovascular cavity of Cnidarians. The term is also used for the analogous inner epithelial layer of Ctenophores. It has been shown that the gastrodermis is amo ...
. In some cases, the mesoglea is noncellular. * In
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s, the mesenchyme is called
mesohylThe mesohyl, formerly known as mesenchyme Mesenchyme () is a type of connective tissue found mostly during the embryonic development of bilateral triploblast animals. Vertebrates Structure Mesenchyme is characterized morphologically by a promi ...
. * In diploblasts (Cnidaria and Ctenophora), the mesenchyme is fully ectodermally derived. This kind of mesenchyme is called ectomesodermal, and is not considered true
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

mesoderm
. * In triploblastic acoelomates (such as
flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

flatworm
s), the term
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the Ancient G ...

parenchyma
is sometimes used for the middle (mesenchymal) layer, in which the dense layer includes tissues derived from both ectoderm, and entomesoderm (true mesoderm, derived from entoderm). When cellular material is sparse or densely packed, as in cnidarians, the mesenchyme may sometimes be called
collenchyma The ground tissue of plants includes all tissues that are neither dermal The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: pro ...
, or parenchyma in flatworms. When no cellular material is present as in
Hydrozoa Hydrozoa (hydrozoans; ) are a taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification sch ...

Hydrozoa
), the layer is properly called
mesoglea Mesoglea refers to the tissue found in cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical num ...
. In some
colonial Colonial or The Colonial may refer to: * Colonial, of, relating to, or characteristic of a colony or colony (biology) Architecture * American colonial architecture * French Colonial * Spanish Colonial architecture Automobiles * Colonial (1920 auto ...
cnidarians, the mesenchyme is perforated by gastrovascular channels continuous among colony members. This entire matrix of common basal material is called coenenchyme.


References

* {{Authority control Connective tissue